In connection with an advance of an information society and an exchange of an enormous amount of information, a recording apparatus for recording a high capacity information becomes common, and a recording density of a recording medium is significantly improved. In general, in the optical recording apparatus of the CD, the DVD or the like, a physical limitation of the recording density is determined on the basis of a used laser wave length and a numerical aperture of an objective lens. Accordingly, an improvement of the recording density is intended by making the laser wave length shorter and making the numerical aperture of the objective lens larger. However, since a technical standard of the optical system of the recording and reproducing apparatus, the recording medium or the like is changed to a new one each time when the recording density is improved in accordance with this method, a great cost is required for developing the recording medium having the improved recording density and the recording and reproducing apparatus thereof so as to manufacture and sell.
On the other hand, it is considered to intend an increase of the recording capacity by using a technique such as a red DVD which has been already marketed and has been inexpensive. For example, in patent document 1, there is proposed a method utilizing a thin substrate which has an improved mechanical property, is inexpensive and can be easily manufactured. In an embodiment in the patent document 1 (JP-B2-7-3690), there is described a structure in which an optical disc is manufactured by pinching a disc having a recording film formed on a substrate made of a polyether sulfone resin and having a thickness of 100 μm between two acrylic discs having a thickness of 1.5 mm and a diameter of 200 mm at a high pressure, and the optical disc is mounted on a turn table and is optically read and written.
However, in the technique described in the patent document 1, since the optical disc having the diameter of 200 mm is utilized as the recording medium, there are problems that the apparatus becomes large in size and complicated in comparison with the existing DVD recording and reproducing apparatus or the like, and a manufacturing cost is increased. Further, in order to achieve a higher capacity, it is necessary to further enlarge the size of the optical disc.
Accordingly, there has been proposed to use a thin optical disc having a thickness of about 100 μm in which a recording film is pinched by thin transparent sheets in place of an acrylic material, the thin optical disc corresponding to a recording medium having a diameter of about 120 mm in the same manner as the existing CD, DVD or the like. If a cartridge type recording medium is structured by receiving a plurality of thin optical discs mentioned above, it is possible to increase a recording capacity without making the recording and reproducing apparatus large in size and complicated.
Patent document 2 (JP-A-10-308059) describes a recording and reproducing mechanism holding and rotating the thin optical disc as mentioned above on a turn table. In this recording and reproducing mechanism, the recording and reproduction can be executed by using the same optical system as that of the existing CD, DVD or the like, on the basis of a provision of an optical correction means having an equal optical path length to the acrylic substrate used in the conventional DVD recording medium or the like, between the optical system and the thin optical disc.